Low visibility watermarks using an out-of-phase color

ABSTRACT

The present invention relates to digital watermarks. In a preferred embodiment, a media signal is embedded with a digital watermark component. The media signal includes a cyan color plane, a magenta color plane, a yellow color plane, and a black plane. The digital watermark component is embedded in the cyan, magenta, and yellow color planes. The digital watermark component is inverted, and embedded in the black color plane. The resulting watermark is fragile, since signal processing techniques that combine the color planes with the black color plane effectively cancels the watermark signal in local areas. The inventive watermark also includes low-visibility properties, by canceling perceived luminance change in local areas throughout the media signal. In another embodiment, a watermark signal is embedded in a first color scheme to be out-of-color gamut in a second color scheme.

RELATED APPLICATION DATA

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/553,084, entitled “Color Adaptive Watermarking,” filed Apr.19, 2000. This application is also related to U.S. patent applicationSer. No. 09/503,881, filed Feb. 14, 2000, which is a continuation inpart of application Ser. No. 09/186,962, filed Nov. 5, 1998, which is acontinuation of application Ser. No. 08/649,419, filed May 16, 1996, nowU.S. Pat. No. 5,862,260. Application Ser. No. 08/649,419 is acontinuation in part of PCT/US96/06618, filed May 7, 1996, U.S.application Ser. No. 08/637,531, filed Apr. 25, 1996 (now U.S. Pat. No.5,822,436), U.S. application Ser. No. 08/534,005, filed Sep. 25, 1995,(now U.S. Pat. No. 5,832,119), and U.S. application Ser. No. 08/436,102,filed May 8, 1995, (now U.S. Pat. No. 5,748,783).

FIELD OF THE INVENTION

The present invention relates to digital watermarking systems andmethods, and is particularly illustrated with reference to fragile andlow-visibility watermarks.

BACKGROUND AND SUMMARY OF THE INVENTION

In color image processing applications, it is useful to understand howhumans perceive colors. By understanding the human visual system and itssensitivity to certain colors, one can more effectively create andmanipulate images to create a desired visual effect. This assertion isparticularly true in image processing applications that intentionallyalter an image to perform a desired function, like hiding information inan image or compressing an image. In digital watermarking, for example,one objective is to encode auxiliary information into a signal, such asan image or video sequence, so that the auxiliary information issubstantially imperceptible to humans in an output form of the signal.

Digital watermarking technology, a form of steganography, encompasses agreat variety of techniques by which plural bits of digital data arehidden in some other object, preferably without leaving human-apparentevidence of alteration.

Digital watermarking may be used to modify media content to embed amachine-readable code into the media content. The media may be modifiedsuch that the embedded code is imperceptible or nearly imperceptible tothe user, yet may be detected through an automated detection process.

Most commonly, digital watermarking is applied to media signals such asimages, audio, and video signals. However, it may also be applied toother types of data, including documents (e.g., through line, word orcharacter shifting, through texturing, graphics, or backgrounds, etc.),software, multi-dimensional graphics models, and surface textures ofobjects.

There are many processes by which media can be processed to encode adigital watermark. Some techniques employ very subtle printing, e.g., offine lines or dots, which has the effect slightly tinting the media(e.g., a white media can be given a lightish-green cast). To the humanobserver the tinting appears uniform. Computer analyses of scan datafrom the media, however, reveals slight localized changes, permitting amulti-bit watermark payload to be discerned. Such printing can be by inkjet, dry offset, wet offset, xerography, etc.

The encoding of a document can encompass artwork or printing on thedocument, the document's background, a laminate layer applied to thedocument, surface texture, etc. If a photograph or image is present, ittoo can be encoded.

Printable media—especially for security documents (e.g., banknotes) andidentity documents (e.g., passports)—is increasingly fashioned fromsynthetic materials. Polymeric films, such as are available from UCBFilms, PLC of Belgium, are one example. Such films may be clear andrequire opacification prior to use as substrates for security documents.The opacification can be affected by applying plural layers of ink orother material, e.g., by gravure or offet printing processes. (Suitableinks are available, e.g., from Sicpa Securink Corp. of Springfield, Va.)In addition to obscuring the transparency of the film, the inks appliedthrough the printing process form a layer that is well suited tofine-line printing by traditional intaglio methods. Such an arrangementis more particularly detailed in laid-open PCT publication WO98/33758.

Digital watermarking systems typically have two primary components: anembedding component that embeds the watermark in the media content, anda reading component that detects and reads the embedded watermark. Theembedding component embeds a watermark pattern by altering data samplesof the media content. The reading component analyzes content to detectwhether a watermark pattern is present. In applications where thewatermark encodes information, the reading component extracts thisinformation from the detected watermark. Previously mentioned U.S.patent application Ser. No. 09/503,881, filed Feb. 14, 2000, disclosesvarious encoding and decoding techniques. U.S. Pat. Nos. 5,862,260 and6,122,403 disclose still others. Of course, artisans know many otherwatermarking techniques that may be suitably interchanged with thepresent invention.

One form of digital watermarks is a so-called “fragile” watermark. Afragile watermark is designed to be lost, or to degrade predictably,when the data set into which it is embedded is processed in some manner,such as signal processing, scanning/printing, etc. A watermark may bemade fragile in numerous ways. One form of fragility relies on lowwatermark amplitude. That is, the strength of the watermark is onlymarginally above the minimum needed for detection. If any significantfraction of the signal is lost, as typically occurs in photocopyingoperations, the watermark becomes unreadable. Another form of fragilityrelies on the watermark's frequency spectrum. High frequencies aretypically attenuated in the various sampling operations associated withdigital scanning and printing. Even a high amplitude watermark signalcan be significantly impaired, and rendered unreadable, by suchphotocopying operations. (Fragile watermark technology and variousapplications of such are even further disclosed, e.g., in assignee'sU.S. patent application Ser. Nos. 09/234,780, 09/433,104, 09/498,223,No. 60/198,138, Ser. Nos. 09/562,516, 09/567,405, 09/625,577,09/645,779, and No. 60/232,163.).

The present invention discloses a new fragile watermarking techniquethat is particularly well suited for color imaging applications. Awatermark signal in one color plane (or channel) is applied to be out ofphase with corresponding watermark signals in other color planes (orchannels). An effect of the inventive out-of-phase watermarkingtechnique is to greatly reduce watermark visibility by cancelingperceived luminance change in local areas throughout the image. Thedisclosed watermark is also fragile, since signal-processing operationsthat combine the out-of-phase color channel with the other channelscancels the watermark signal.

The foregoing and other features and advantages of the present inventionwill be more readily apparent from the following detailed description,which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a color space depicting how to scale a colorvector to black to effect a change in luminance.

FIG. 2 is a diagram of a color space depicting how to scale a colorvector to white to effect a change in luminance.

FIG. 3a is a diagram illustrating color data for a mid-gray image patch.

FIG. 3b illustrates the color data of FIG. 3a, embedded with a digitalwatermark signal.

FIG. 4 is a flow diagram illustrating an embedding process.

FIGS. 5 and 6 are diagrams regarding obtaining luminance values.

FIG. 7 is a flow diagram illustrating a detection method.

FIG. 8 is a flow diagram illustrating a detection method of data in aRGB color space.

FIG. 9 illustrates approximating a color saturation value in an RGBcolor space.

FIG. 10 is a flow diagram illustrating a luminance collection methodaccording to the present invention.

FIGS. 11 and 12 are diagrams illustrating relative threshold values forthe method shown in FIG. 8.

FIG. 13 is a diagram illustrating RGB and CMY common color gamuts, andout-of-gamut colors.

DETAILED DESCRIPTION

Introduction

A watermark can be viewed as an information signal that is embedded in ahost signal, such as an image, audio, video or some other media content.Watermarking systems typically include the following components: 1) anembedder that inserts a watermark signal in the host signal to form acombined signal; 2) a detector that determines the presence andorientation of a watermark in a potentially corrupted version of thecombined signal; and 3) a reader that extracts a watermark message fromthe combined signal. In some implementations, the detector and readerare combined.

To encode a message, the watermark encoder analyzes and selectivelyadjusts the host signal to give it attributes that correspond to adesired message symbol or symbols to be encoded. There are many signalattributes that may encode a message symbol, such as a positive ornegative polarity of signal samples or a set of samples, a given parity(odd or even), a given difference value or polarity of the differencebetween signal samples (e.g., a difference between selected spatialintensity values or transform coefficients), a given distance valuebetween watermarks, a given phase or phase offset between differentwatermark components, a modulation of the phase of the host signal, amodulation of frequency coefficients of the host signal, a givenfrequency pattern, a given quantizer (e.g., in Quantization IndexModulation), etc.

The structure and complexity of a watermark signal can varysignificantly, depending on the application. For example, the watermarkmay be comprised of one or more signal components, each defined in thesame or different domains. Each component may perform one or morefunctions. Two primary functions include acting as an identifier tofacilitate detection and acting as an information carrier to convey amessage. In addition, components may be located in different spatial ortemporal portions of the host signal, and may carry the same ordifferent messages.

The host signal can vary as well. The host is typically some form ofmulti-dimensional media signal, such as an image, audio sequence orvideo sequence. In the digital domain, each of these media types isrepresented as a multi-dimensional array of discrete samples. Forexample, a color image has spatial dimensions (e.g., its horizontal andvertical components), and color space dimensions (e.g., CMYK, YUV orRGB). Some signals, like video, have spatial and temporal dimensions.Depending on the needs of a particular application, the embedder mayinsert a watermark signal that exists in one or more of thesedimensions.

In the design of the watermark and its components, developers are facedwith several design issues such as: the extent to which the mark isimpervious to jamming and manipulation (either intentional orunintentional); the extent of imperceptibility; the quantity ofinformation content; the extent to which the mark facilitates detectionand recovery, and the extent to which the information content can berecovered accurately.

For certain applications, such as copy protection or authentication, thewatermark should be difficult to tamper with or remove by those seekingto circumvent it. To be robust, the watermark should withstand routinemanipulation, such as data compression, copying, linear transformation,flipping, inversion, etc., and intentional manipulation intended toremove the mark or make it undetectable. Some applications require thewatermark signal to remain robust through digital to analog conversion(e.g., printing an image or playing music), and analog to digitalconversion (e.g., scanning the image or digitally sampling the music).In some cases, it is beneficial for the watermarking technique towithstand repeated watermarking.

For other applications, such as forensic tracking, counterfeitdetection, etc., the watermark should degrade predictably under routinemanipulation. Such watermarks are refereed to generally as “fragile”watermarks, as discussed above.

A variety of signal processing techniques may be applied to address someor all of these design considerations. One such technique is referred toas spreading. Sometimes categorized as a spread spectrum technique,spreading is a way to distribute a message into a number of components(chips), which together make up the entire message. Spreading makes themark more impervious to jamming and manipulation, and makes it lessperceptible.

Another category of signal processing technique is error correction anddetection coding. Error correction coding is useful to reconstruct themessage accurately from the watermark signal. Error detection codingenables the decoder to determine when the extracted message has anerror.

Another signal processing technique that is useful in watermark codingis called scattering. Scattering is a method of distributing the messageor its components among an array of locations in a particular transformdomain, such as a spatial domain or a spatial frequency domain. Likespreading, scattering makes the watermark less perceptible and moreimpervious to manipulation.

Yet another signal processing technique is gain control. Gain control isused to adjust the intensity of the watermark signal. The intensity ofthe signal impacts a number of aspects of watermark coding, includingits perceptibility to the ordinary observer, and the ability to detectthe mark and accurately recover the message from it.

Gain control can impact the various functions and components of thewatermark differently. Thus, in some cases, it is useful to control thegain while taking into account its impact on the message and orientationfunctions of the watermark or its components. For example, in awatermark system described below, the embedder calculates a differentgain for orientation and message components of an image watermark.

Another useful tool in watermark embedding and reading is perceptualanalysis. Perceptual analysis refers generally to techniques forevaluating signal properties based on the extent to which thoseproperties are (or are likely to be) perceptible to humans (e.g.,listeners or viewers of the media content). A watermark embedder cantake advantage of a Human Visual System (HVS) model to determine whereto place an image watermark and how to control the intensity of thewatermark so that chances of accurately recovering the watermark areenhanced, resistance to tampering is increased, and perceptibility ofthe watermark is reduced. Similarly, audio watermark embedder can takeadvantage of a Human Auditory System model to determine how to encode anaudio watermark in an audio signal to reduce audibility. Such perceptualanalysis can play an integral role in gain control because it helpsindicate how the gain can be adjusted relative to the impact on theperceptibility of the mark. Perceptual analysis can also play anintegral role in locating the watermark in a host signal. For example,one might design the embedder to hide a watermark in portions of a hostsignal that are more likely to mask the mark from human perception.

Various forms of statistical analyses may be performed on a signal toidentify places to locate the watermark, and to identify places where toextract the watermark. For example, a statistical analysis can identifyportions of a host image that have noise-like properties that are likelyto make recovery of the watermark signal difficult. Similarly,statistical analyses may be used to characterize the host signal todetermine where to locate the watermark.

Each of the techniques may be used alone, in various combinations, andin combination with other signal processing techniques.

In addition to selecting the appropriate signal processing techniques,the developer is faced with other design considerations. Oneconsideration is the nature and format of the media content. In the caseof digital images, for example, the image data is typically representedas an array of image samples. Color images are represented as an arrayof color vectors in a color space, such as RGB or YUV. The watermark maybe embedded in one or more of the color components of an image. In someimplementations, the embedder may transform the input image into atarget color space, and then proceed with the embedding process in thatcolor space.

Color Image Processing

In image processing applications, it is sometimes useful to be able tochange the colors of an image while reducing the visibility of thesechanges. Image watermarking is one application where it is desirable toalter image samples to encode information in a manner that is readilyrecoverable by an automated process, yet substantially imperceptible tohuman visual perception. Often, the aim of watermark encoding is tomaximize a watermark signal without significantly affecting imagequality. Since the eye is more sensitive to changes in memory colorssuch as flesh tones or blue sky, it is beneficial to have a method ofselectively controlling strength of a watermark in certain colorregions. Previously mentioned U.S. patent application Ser. No.09/553,084 discloses various methods for such.

In one embodiment, a watermark encodes auxiliary information in an imageby making changes to image samples. A color-masking framework maps achange in an image sample attribute to an equivalent yet lessperceptible change in the color values of that image sample. Thismapping can be used to obtain equal perceptual watermark changes toimage samples in other areas of color space and to apply the change inthe least visible color channels.

While the implementation details of watermark encoding schemes varysignificantly, a class of watermarking schemes can be modeled as anarray of changes to luminance values of a host image. The host imagecomprises an array of color vectors (e.g., an array of color such asRGB, CMY, CMYK, etc). The image sample may be represented as a vectorbetween black and the pixel color value. To encode a watermark, theluminance of the image sample may be increased or decreased as shown inFIG. 1. FIG. 1 shows a 3-dimensional color space with Cyan (C), Magenta(M) and Yellow (Y) axes. The bold axis between black and whiterepresents luminance. To make an equivalent luminance change in an imagesample of a given color vector (C1, M1, Y1), one may make acorresponding scale to black as shown.

An alternative method of obtaining the same luminance change is to scalethe image sample like a vector between white and the sample's colorvalue as shown in FIG. 2. To make an equivalent luminance change, onemay make a corresponding scale to white as shown.

By using the scale to white method for colors with high yellow contentsuch as yellow, red and green, and scale to black for blue, cyan andmagenta a lower visibility watermark can be encoded with the samedetectability.

Once the color vector entries are established, each of the entries isassociated with a set of scale factors. The set includes a scale factorfor each color component. The specific color components in theimplementation depend on the color format of the image. For example,images in an RGB format have scale factors for each of the R, G and Bcolor components. Similarly, images in a CMY format have scale factorsfor each of the C, M and Y components of each table entry. The scalefactors for each entry are derived by rewriting the above mathematicalexpression and solving for each color's scale factor as a function ofthe known color component values.

Low Visibility Watermarks Using an Out-of-Phase Color

Three-color components, such as Red, Green, Blue (RGB) or Luminance,color component “a” and color component “b” (Lab), can be combined touniquely identify a particular color. In many cases, more thanthree-color components can be combined to specify (or approximate) thesame particular color. Typically, four (4) color components, e.g., cyan,magenta, yellow and black (CMYK) are used in printing processes. Extracolors, sometimes referred to as “spot colors,” can be added for moreaccurate color reproduction. A company logo, might include a particularshade of green, which is printed with a corresponding green ink (e.g., agreen spot color). High fidelity color printing often uses more thanfour (4) color components. These additional color components expand thegamut of printing colors for very high quality printing, such as fineart reproduction. Security printing (e.g., bank notes, financialdocuments, certificates, etc.) also uses a plurality of inks, with finelines, such as intaglio.

Printing processes with more than three (3) inks (or color componentdimensions) have a built in redundancy, since more than three (3) colorcomponents (e.g., CMYK) are used to specify a color at a particularpoint in an image. This implies that many different combinations of four(4) or more printing inks (or components) can be used to represent athree (3)-component color. By way of example only, a point in the CMYcolor space (e.g., 61% cyan, 50% magenta, and 48% yellow) can berepresented (or approximated) by a corresponding point in the CMYK colorspace (e.g., 51% cyan, 40% magenta, 38% yellow, and 22% black). Thissame 3-channel color point can also be represented in the CMYK colorspace as 32% cyan, 23% magenta, 22% yellow, and 43% black. Of course,this is but one of many possible color combinations. One aspect of thepresent invention utilizes this color redundancy, to reduce the humanvisibility of a digital watermark. For example, a watermark signal inone color channel, can be effectively counteracted (e.g., cancelled) inthe other color (or black) channels, while still obtaining the desiredcolor.

Another aspect of the present invention provides a fragile watermark.The watermark's fragility is due, at least in part, to its small-scalespatial variation within a media signal, making the watermark ideal tocombat typical counterfeiting operations such as scanning/printing,digital manipulation and photocopying. Of course, the process describedfor “CMYK” media below, could also be applied in a similar manner toother multi-color plane printing processes.

Watermark Embedding

Media is embedded with a watermark signal. Of course, the media maycorrespond to an image, digital image, photograph, video frame, graphic,etc., and in some cases, may even include a physical object such as adocument, banknote, postage stamp, etc. Typically, a watermark signalhas at least one component, which when embedded in the media correspondsto (or affects) various areas (or pixels) in the media. In the case ofan area (or pixel) represented in a color space, the watermark signalcomponent can be added to (or subtracted from) the point in all or someof the color dimensions.

Consider FIG. 3a, where the dash/dot C, M, Y and K lines representrespective cyan, magenta, yellow and black color dimensions for a linein a mid-gray patch of a media signal (e.g., a line in a picture, image,document, etc.). FIG. 3b illustrates the media of FIG. 3a, which hasbeen embedded with a watermark signal. The watermark signal ispreferably applied to each of the color component dimension (C, M, andY). In FIGS. 3a and 3 b, the M and Y channels are represented by onesignal, since these color components can be approximately equal, butseparate signals for gray. Of course, it is not necessary for thesecomponents to be equal, and in many cases the yellow and magentacomponents are not equal. The illustrated embedded “bumps” in FIG. 3brepresent the watermark signal, e.g., upward or downward signaladjustments in relation to the respective color channel at given pointsover the media line. For the K dimension (or channel), the watermarksignal is preferably embedded to be out-of-phase with the CMY channels.Most preferably, the K channel is approximately 180 degrees out-of-phase(e.g., inverted) with the watermark signals in the CMY color channels,as shown in FIG. 3b.

With reference to FIG. 4, one out-of-phase embedding method generates(or determines) a watermark signal as shown in step S1. The watermarksignal is embedded in the CMY channels in step S2. The inverse of thewatermark signal is calculated in step S3, and such inverse signal isembedded in the K channel in step S4. Of course, the order of such stepsis not critical. For example, a watermark signal can be determined, andan inverse calculated. The various color component dimensions can thenbe embedded. In another embodiment, the K channel is embedded with awatermark signal. An inverse signal is calculated, and the CMY channelsare embedded with the inverse signal.

Such an inventive watermarking scheme greatly reduces watermarkvisibility. Since the watermark signal for the K channel is appliedapproximately 180 degrees out of phase, when compared to the respectivechanges applied to the CMY channels, the watermark visibility is greatlyreduced. The visibility reduction is produced by the effectivecancellation of perceived luminance changes, when the CMYK image isviewed or printed. Indeed, combining an inverted watermark signal“tweak” or “bump” in a K channel, with a corresponding non-invertedwatermark signal tweak in the CMY channels effectively cancels anoverall perceived luminance change—effectively reducing visibility ofthe digital watermark.

Watermark Detection

Another aspect of the present invention is a detection method and systemfor detecting an out-of-phase, color component-based digital watermark.Consider a first embodiment as depicted in FIGS. 5 and 6. A media signalis analyzed as follows. Initially, luminance values are determined forCMY and K color planes. The CMY luminance can be computed as a properlyweighted sum of the cyan, magenta and yellow primary components. Forexample: Lum=0.3C+0.6M+0.1Y. (Of course, as will be appreciated by thoseskilled in the art, other weighting coefficients can be used todetermine a CMY luminance.). So for a given point (or pixel), aluminance value is determined for the CMY color dimensions. Similarly,as shown in FIG. 6, a luminance value for K is determined. In oneembodiment, the K luminance equals the value of the K component.

The detection process is further described with respect to FIG. 7. Instep S10, CMY and K are converted to approximate luminance values perpixel (or area):

 CMY=>lum _(cmy)

K=>lum _(k),

where lum_(cmy) is the luminance of CMY and lum_(k) is the luminance ofK. In step S11, lum_(k) is subtracted from lum_(cmy):

Lum _(final) =lum _(cmy) −lum _(k).

The step S11 subtraction operates to help reduce image content, and toreinforce the watermark signal by effectively adding the K watermarksignal value to the CMY watermark signal, since the K watermark signalis the inverse of the CMY channel signals.

As shown in step S12, Lum_(final) can be analyzed for watermarkdetection.

Fragile Watermark

An out-of-phase watermark is fragile since a signal processing operationthat combines the K channel with the CMY channel effectively cancels thewatermark signal. Conversion to other color spaces similarly degradesthe watermark signal. Take a typical scan/print process for example.Digital scanners typically convert scanned images into a RGB colorscheme. Scanning an out-of-phase embedded CMYK image degrades theembedded watermark due to the combination of K with CMY in a local area.When the RGB image representation is printed, the watermark signal isdifficult to detect, particularly with a low resolution RGB scan.Similarly, other conversions, such as to a Lab color space, degrade theout-of-phase watermark due to the combination of K with CMY throughoutlocal areas. Nevertheless, the watermark signal is detectable with CMYKdata as described above with respect to FIG. 7.

A fragile watermark has utility in many applications. Takecounterfeiting, for example. The inventive fragile watermark is embeddedin original CMYK media. If the media is copied, the embedded fragilewatermark is either lost or degrades predictable. The copy is recognizedas a copy (or counterfeit) by the absence or degradation of the fragilewatermark. Fragile watermarks can also be used in conjunction with otherwatermarks, such as robust watermarks. The fragile watermark announces acopy or counterfeit by its absence or degradation, while the otherrobust watermark identifies author, source, links and/or conveysmetadata or other information, etc. In other embodiments, a fragilewatermark is an enabler. For example, some fragile watermark may includeplural-bit data that is used to enable a machine, allow access to asecure computer area, verify authenticity, and/or link to information.This plural-bit data is lost or sufficiently degrades in a copy,preventing the enabling functions. Other fragile watermark applicationsare discussed in the U.S. patent applications incorporated above.

High Resolution Scan of Watermarked Image

Oftentimes, CMYK data may not be available for a detection process. Forexample, a watermarked CMYK image may be optically scanned with ascanner that converts the CMYK data into a different color space, suchas to RGB. A high resolution RGB scan may nevertheless be used torecover an estimation of the watermark signal, which would be otherwiseundetectable with a low RGB resolution scan. In this case, pixels can beassigned color or K values to generate respective color and K planes. Afinal luminance value can be determined from these planes. Consider thefollowing method, as shown in FIG. 8, for a high resolution RGB scan(e.g., about 8 times the screen ruling or more) of a CMYK image.

In step S20, a color saturation (ColSat) value is determined for eachRGB pixel. ColSat can be calculated by dropping a perpendicular linefrom a measured RGB value (e.g., R₀, G₀, B₀, as shown in FIG. 9) to theRGB luminance axis. This color saturation calculation alternatively canbe approximated by:

 ColSat=max(RGB)−min(RGB),

where max(RGB) is determined by taking the maximum of the red, green andblue values R₀, G₀, B₀, and min(RGB) is determined by taking the minimumof the red, green and blue values R₀, G₀, B₀. Of course, other knownmethods for determining a color saturation value are suitablyinterchangeable with this step.

In step S22, preferably for each RGB pixel (or pixel area), it isdetermined whether ColSat<T_(col), where T_(col) is a predeterminedthreshold color saturation, e.g., based on a scanner calibration,detection sensitivity and/or other threshold number. For example, a purecyan pixel will typically correspond to a (0, 100, 100) RGB value.However, a scanner may read this pixel as a (20, 80, 80) RGB value. Thethreshold value T_(col) can be set to allow for such scannersensitivity. T_(col) can also be adjusted to reflect acceptabletolerances. If ColSat<T_(col), flow continues to step S24, where it isdetermined whether the maximum measured (e.g., determined, scanned,calculated, etc.) pixel value is <T₁. Here, T₁ is a predetermined pixelvalue, e.g., based on scanner calibration, detection sensitivity and/orother threshold numbers. FIG. 11 illustrates a graphical relationshipthat shows digital threshold and scanner pixel values on a scale betweenBlack (0) and white (255). Value S_(B) is a scanner measured (ordesignated) black, which due to scanner sensitivity and/orcharacteristics is generally offset from black (0). Similarly, SCMY is ascanner measured CMY overprint (e.g., a 100% overprint). If the maximumpixel value is<T₁ the pixel is designated as black (K) in step S26. Acorresponding pixel range is shown graphically in FIG. 11, as the valuesless than T₁. As discussed, T₁ can be selected based on scannercharacteristics. In one embodiment, T₁ is selected to have a valuemidway between scanner-measured black (S_(B)), and scanner measured CMYoverprint (S_(CMY)). Of course, T₁ can be adjusted from this midpointvalue to accommodate sensitivity requirements and/or scannercharacteristics. (A maximum pixel value can be chosen in a number ofknown techniques, such as selecting the color component of a pixel,e.g., if a measured pixel corresponds to (20, 70, 80), the colorcomponent 80 comprises the maximum pixel value.).

If the max pixel value is not<T₁, the pixel value is compared againstanother threshold value. In step S28, it is determined whether the maxpixel value is<T₂. If so, the pixel is designated as color in step S30.Returning again to FIG. 11, the corresponding range of pixel valuesfalls between T₁ and T₂. The threshold value T₂ can be selected based onscanner characteristics, or based on sensitivity requirements. In oneembodiment, T₂ is selected to have a value midway between SCMY and white(255). Of course, this value can be adjusted based on sensitivity needand/or scanner characteristics.

If the max pixel value is not<T₂, the pixel value is designated as whitein step S32. The corresponding pixel range lies between T₂ and white(255).

If in step S22, it is determined that ColSat is not<T_(col), flowcontinues to step S34, where it is determined whether the max pixelvalue is<T₃. If so, the pixel is designated as a color pixel in stepS36. Here, T₃ is a predetermined pixel value. In one embodiment, T₃ isselected to have a value midway between a scanner measured (ordetermined) yellow value and white (255). Of course, this value can beadjusted based on sensitivity requirements and/or scannercharacteristics. Otherwise, the pixel value is designated white in stepS38. This relationship is shown in FIG. 12, where S_(C), S_(M) and S_(Y)corresponding with scanner measured (or determined) cyan, magenta andyellow values.

Color and K planes can be constructed once each RGB color pixel isdesignated as a color, white or K pixel.

To create the K plane, pixels designated as black are turned “on,” whilethe pixels that are designated as white or color are turned “off.” Inone embodiment, the respective “off” pixels are masked. In anotherembodiment, the off pixels (or alternatively the on pixels) are flaggedor otherwise marked to indicate their designation and/orinclusion/non-inclusion.

Similarly, to create the color plane, pixels designated as “white” or“black” are turned off, while the rest of the pixels (e.g., the colorpixels) are turned on.

The pixel (or area) values are summed for the color plane to obtain alow resolution (LR) color luminance (lum_(color)) per pixel (or perarea). Similarly, the pixel values are summed for the black plane toobtain a LR black luminance (lum_(K)). A final luminance value(lum_(final)) for each pixel (or area) can be determined from:

lum _(final) =lum _(color) −lum _(k).

The lum_(final) value can be passed into a watermark detection process.

FIG. 10 illustrates one possible summing method for each color (colorand K) plane according to a preferred embodiment to achieve the abovecolor and black luminance values.

In step S40, each of the luminance values within a predetermined window(or pixel area) are summed. The window may be an n x n window, or and nx m window, where n and m are integers. In a preferred embodiment,pixels values with an 8×8 pixel window are summed. The resultingsummation value is preferably saved for comparison against the otherplane (color or K), to determine lum_(final) as discussed above.

In step S42, the window location with respect to the color (or K) planeis repositioned (e.g., the window is a sliding window). To illustrate,if a first window frames the first n×n pixels in a color (or K) plane,the second window is adjusted to cover a new area, or an overlappingarea. In the preferred embodiment, the window slides right (orhorizontally) by four (4) pixels (e.g., on a first slide, the window nowcovers the 5^(th)-12^(th) pixels×8). The luminance values within thissecond window are added in step S44. This value is preferably saved forcomparison against the other color plane (or K plane), to determinelum_(final) as discussed above.

The method determines if the window is at the end of a line (e.g., atthe end of a plane edge) in step S46. If not, flow continues to stepS42, where the window location is again repositioned. Otherwise, it isdetermined whether the entire plane has been analyzed in step S48. Ifso, the method ends. Otherwise, the window location is repositioned instep S50. The step S50 location adjustment preferably moves the windowlocation down (or vertically) with respect to the plane to cover a newarea or an overlapping area. In the preferred embodiment, the window isshifted down by 4 pixels. At this step, it is important to note that thenew window location need not be realigned at the staring point (e.g.,the top-left corner of the plane), but may be shifted down at the rightplane edge. The window may then slide right to left. Of course, the stepS50 alignment may locate the window below the original starting point(e.g., the left plane edge), with the window sliding left to right. Flowcontinues from step S50 to S40.

This process is preferably carried out for each of the color and Kplanes. The resulting area black luminance values are subtracted fromthe corresponding resulting color luminance values to achieve the finalluminance value. This final luminance value (for each location area) canbe analyzed to detect the watermark signal.

Embedding in Out-of-Range Colors

Another inventive fragile watermarking technique embeds watermark datain out-of-range colors. A color gamut defines a range of colors.Different color schemes (e.g., RGB and CMY) generally include a uniquecolor gamut. Such color schemes will most certainly have overlappingcolor gamuts (or ranges), and unique (or out of gamut) color ranges, asshown in FIG. 13.

Differences in gamut between color models can be used to indicate that atransformation (or copy) has occurred. Consider security printers, whichoften select inks that lie outside the common color gamuts of capturedevices (RGB) when printing documents. Printing with such out-of-range(or out of gamut) colors makes counterfeiting even more difficult.Consider a document that is printed with some dark blues & violets inthe CMYK space, which are out of gamut for the RGB space. When a scannerscans the CMYK document, it typically converts the scanned image intothe RGB space. Such processing looses the dark blues and violets in theconversion. An educated inspector can identify a counterfeit document bylooking for the presence (or absence) of certain colors.

The inventive fragile watermark utilizes out-of-range color gamuts. Takethe example given above. Dark blues & violets in the CMY (or CMYK) spaceare out of gamut with respect to the RGB space. Accordingly, a mask (orcolor spectral analysis) is used to identify dark blues and violets in amedia signal. These areas are used (e.g., masked) as the areas forembedding watermark signals. The watermark is detectable with a CMYdetection process. However, if the document is scanned with a RGBelement scanner, the watermark is generally lost. As discussed above,conversion from CMYK to RGB fails to accurately convert out-of-phasecolors. In this case, the dark blues and violets are out-of-gamut.Accordingly, since the watermark signal is embedded in the out-of-gamutcolors, it is lost (or predictably degraded) during the conversion fromCMY to RGB.

Consider the possibilities of such a fragile watermark. One can track,trace, detect counterfeits, control enabling functions, and many, manymore application, like those discussed above.

Concluding Remarks

The foregoing are just exemplary implementations of the presentinvention. It will be recognized that there are a great number ofvariations on these basic themes. The foregoing illustrates but a fewapplications of the detailed technology. There are many others.

It should be appreciated that the order of steps in the FIGS. 4, 8, and10 flow charts can be reordered without deviating from the scope of thepresent invention. For example, in FIG. 4, the K channel could beembedded first, and then the CMY channels. Or the K channel could beembedded concurrently with or in between the color channels. In FIG. 8,instead of basing the step S22 decision on color saturation, the stepS22 decision could be based on whether the pixel value is above or belowone or all of the predetermined thresholds. The color saturation couldbe analyzed in subsequent steps. Also, the signs of the decisions can bereversed, which will respectively reverse the decision tree branches,and color ranges. Also, decisions could be based on whether a pixelvalue is less than, or equal to a threshold value. Moreover, instead ofa maximum pixel value, an average pixel value, or lower pixel valuecould be used. In FIG. 10, the window could alternatively be firstvertically repositioned, and then horizontally. The window can also berepositioned on a random, or pseudo-random basis. The window size mayalso be varied. Also whereas the FIGS. 3a and 3 b illustrate a mid-graypatch, the present invention is not so limited. Indeed the scope of thepresent invention covers any set of 2 or more primary colors.

Preferably, an out-of phase watermark signal is embedded 180 degrees outof phase with corresponding channels. However, some cancellation willstill be achieved if the signal is approximately 180 degrees, forexample, in a range of ±0-20 degrees off of the 180-degree mark.

The section headings in this application are provided merely for thereader's convenience, and provide no substantive limitations. Of course,the disclosure under one section heading may be readily combined withthe disclosure under another section heading.

To provide a comprehensive disclosure without unduly lengthening thisspecification, the above-mentioned patents and patent applications arehereby incorporated by reference. The particular combinations ofelements and features in the above-detailed embodiments are exemplaryonly; the interchanging and substitution of these teachings with otherteachings in this application and the incorporated-by-referencepatents/applications are also contemplated.

The above-described methods and functionality can be facilitated withcomputer executable software stored on computer readable media, such aselectronic memory circuits, RAM, ROM, magnetic media, optical media,memory sticks, hard disks, removable media, etc., etc. Such software maybe stored and executed on a general purpose computer, or on a server fordistributed use. Data structures representing the various luminancevalues, summations, out-of-phase embedded signals, embedded colorplanes, color signals, data signals, luminance signals, etc., may alsobe stored on such computer readable media. Also, instead of software, ahardware implementation, or a software-hardware implementation can beused.

In view of the wide variety of embodiments to which the principles andfeatures discussed above can be applied, it should be apparent that thedetailed embodiments are illustrative only and should not be taken aslimiting the scope of the invention. Rather, we claim as our inventionall such modifications as may come within the scope and spirit of thefollowing claims and equivalents thereof.

What is claimed is:
 1. A method of embedding a digital watermarkcomponent in a signal having a plurality of channels, said methodcomprising the steps of: embedding the digital watermark component in afirst of the plurality of channels; and embedding the digital watermarkcomponent in a second of the plurality of channels to be out-of-phasewith respect to the digital watermark component in the first channel. 2.The method according to claim 1, wherein the plurality of channelscomprises color channels and at least one black channel.
 3. The methodaccording to claim 2, wherein the color channels comprise cyan, magentaand yellow.
 4. The method according to claim 3, further comprising thesteps of embedding the digital watermark component in at least a thirdchannel and a fourth channel.
 5. The method according to claim 4,wherein the first channel, third channel and fourth channel respectivelycomprise the cyan, magenta and yellow, and the second channel comprisesblack.
 6. The method according to claim 5, wherein the digital watermarkcomponent in the second channel is embedded approximately 180 degreesout-of-phase with respect to the digital watermark component in thefirst channel.
 7. The method according to claim 1, wherein the digitalwatermark component in the second channel is embedded approximately 180degrees out-of-phase with respect to the digital watermark component inthe first channel.
 8. A method of embedding a media signal with adigital watermark signal, the media signal comprising a first colorplane, a second color plane, a third color plane, and a fourth colorplane, said method comprising the steps of: combining the watermarksignal with the first color plane, second color plane and third colorplane; inverting the digital watermark signal; and combining theinverted digital watermark signal with the fourth color plane.
 9. Themethod of claim 8 wherein the inverted digital watermark signal ascombined in the fourth color plane is spatially registered with thedigital watermark signal as combined in the first color plane, secondcolor plane and third color plane.
 10. The method of claim 8, whereinthe inverted digital watermark signal as combined in the fourth colorplane cancels at least some visual artifacts caused by the combining ofthe digital watermark signal in the first color plane, second colorplane and third color plane.
 11. The method of claim 8, wherein theinverted digital watermark signal as combined in the fourth color planeserves to offset at least some luminance that is attributable to thedigital watermark signal as combined in the first color plane, secondcolor plane and third color plane.
 12. A method of generating a fragilewatermark comprising the steps of: embedding a digital watermark signalin at least a first color plane; and embedding the digital watermarksignal in a second color plane to be out-of-phase with respect to thedigital watermark signal in the first color plane.
 13. The methodaccording to claim 12, further comprising the steps of: embedding thedigital watermark signal in a third color plane; and embedding thedigital watermark signal in a fourth color plane.
 14. The methodaccording to claim 13, wherein the first color plane, third color planeand fourth color plane respectively comprise the cyan, magenta andyellow, and the second color plane comprises black.
 15. The methodaccording to claim 12, wherein the digital watermark in the second colorplane is 180 degrees out-of-phase with respect to the digital watermarkin the first color plane.
 16. A method of conditioning a signalincluding an out-of-phase embedded watermark for watermark detection,the out-of-phase embedded watermark being embedded in a media signalcomprising a plurality of color channels, said method comprising thesteps of: determining a luminance value per pixel for a first set of theplurality of color channels; determining a luminance value per pixel fora second set of the plurality of color channels; and subtracting thesecond set's luminance value from the first set's luminance value. 17.The method according to claim 16, wherein the watermark is embedded in afirst, second and third channel of the plurality of channels, and isembedded to be out-of-phase with the first, second and third channels ina fourth channel.
 18. A method to condition a signal for recovery of anout-of-phase digital watermark which is embedded in a media signal in afirst color scheme, the media signal being subsequently converted to asecond color scheme, said method comprising the steps of: designatingeach pixel in the converted media signal as at least either a firstcolor, a second color or a third color; creating a first color plane anda second color plane based on color designations in said designatingstep; determining a luminance value per area for each of the first colorplane and second color plane; and on an area by area basis, subtractingthe second area's luminance value from the first area's luminance value.19. The method according to claim 18, wherein said designating stepcomprising the step of: determining a designation at least in part bypixel color saturation; and determining the designation at least in partby pixel value.
 20. The method according to claim 18, wherein the firstcolor scheme comprises a CMYK color scheme and the second color schemecomprises an RGB color scheme.
 21. A computer readable media comprisinga media signal including a plurality of channels stored thereon, saidmedia signal having a digital watermark component embedded in at least afirst of the plurality of channels, and the digital watermark componentembedded in a second of the plurality of channels to be approximately180 degrees out-of-phase with respect to the digital watermark componentin the first channel.
 22. The computer readable media according to claim21, wherein the digital watermark signal component is embedded in asecond channel and a third channel of the plurality of channels.
 23. Amethod of embedding a digital watermark in media that includes aplurality of channels, said method comprising the steps of: embedding afirst digital watermark component in a first of the plurality ofchannels; and embedding a second digital watermark component in a secondof the plurality of channels to be inverted relative to the firstdigital watermark component in the first channel.
 24. The method ofclaim 23 wherein the second digital watermark component in the second ofthe plurality of channels is embedded so as to be spatially registeredwith the first digital watermark component in the first of the pluralityof channels when both channels are printed.
 25. The method of claim 23wherein the second digital watermark component in the second of theplurality of channels is embedded so as to cancel at least somevisibility artifacts that are caused by the embedding of the firstdigital watermark component.
 26. The method of claim 23, wherein thesecond digital watermark component in the second of the plurality ofchannels is embedded so as to cancel at least some luminance thatattributable to the first digital watermark component embedded in thefirst of the plurality of channels when both channels are printed. 27.The method of claim 23 wherein the second digital watermark component inthe second of the plurality of channels is embedded so as to bespatially inverted relative to the first digital watermark component inthe first channel.
 28. A method of embedding a digital watermarkcomponent in media that includes a plurality of channels comprising thesteps of: embedding a first digital watermark component in a firstchannel; and embedding a second digital watermark component in a secondchannel, wherein the second digital watermark component is embedded soas to be spatially registered with the first digital watermark componentto reduce at least some visual artifacts that are attributable to thefirst digital watermark component.
 29. The method of claim 28 wherein atleast one of the first channel and the second channel comprises a blackchannel.
 30. A printed document including the embedded media of claim 29printed thereon.
 31. The method of claim 28, wherein the first channelcomprises a cyan channel, a magenta channel and a yellow channel, andthe second channel comprises a black channel.
 32. A printed documentincluding the embedded media of claim 31 printed thereon.
 33. The methodof claim 28, wherein the second channel comprises a cyan channel, amagenta channel and a yellow channel, and the first channel comprises ablack channel.
 34. A printed document including the embedded media ofclaim 33 printed thereon.
 35. A printed document including the embeddedmedia of claim 28 printed thereon.
 36. The method of claim 28, whereinthe second digital watermark component comprises the first digitalwatermark component but in an inverted form, and wherein the visualartifacts comprise a change in luminance that is attributable to thefirst digital watermark component when printed.